In article , Helen
Humphreys writes
How can I get rid of this monster that is trying to take over my whole
garden, spreading from a single clump in just three years? I was naively
ignorant, when I put it in, that it was intent on world domination!! Digging
it out seems futile, with such a fine-grained root system.

Don't, whatever you do, let it seed.

Clipping off the main 'branches' at ground level with secateurs so that
there are no leaves left seems to work eventually. Pulling out small
plants is easy and worthwhile. Otherwise, I suppose you're looking at
weedkiller.
--
Kay
"Do not insult the crocodile until you have crossed the river"